Changes in color Doppler imaging measurements of renal artery blood flow velocity have been reported previously during fetal life and during the first week postnatally in term and preterm infants. This study reports longitudinal, developmental changes in renal artery and aortic blood flow velocities occurring postnatally, from birth to day 1 of life, at 1 week, and at 2 to 3 weeks of age in 14 premature babies (mean gestation, 30 ± 4 [SD] weeks; birth weight, 1.45 ± 0.57 kg), and identified by means of color Doppler imaging and pulsed Doppler spectral analysis. Results indicate that a significant increase in renal artery systolic blood flow velocity occurs within the first week of life (from 40 ± 3 [SEM] cm/sec at birth or on day 1, to 53 ± 3 cm/sec on day 7, to 51 ± 4 cm/sec on day 14 to 21; repeated-measures analysis of variance, p = 0.004), concurrently with a significant increase in abdominal aortic blood flow velocities, both systolic (from 40 ± 4 at birth or on day 1, to 70 ± 8 on day 7, to 76 ± 8 cm/sec on day 14 to 21; p <0.001) and diastolic (from 4 ± 2 at birth or on day 1, to 11 ± 2 on day 7, to 11 ± 2 cm/sec on day 14 to 21; p = 0.001). Systemic blood pressure did not increase concomitantly during the same period. Neither the presence of respiratory distress syndrome or patent ductus arteriosus nor treatment with indomethacin altered developmental increases in observed renal artery blood flow velocities. The presence of an umbilical artery catheter in the high thoracic position in five infants, however, created turbulence at the level of the renal arteries, significantly increasing renal artery systolic flow velocity from 32 ± 4 to 44 ± 5 cm/sec (p = 0.009) and increasing renal resistive index from 0.90 ± 0.03 to 0.96 ± 0.04 (p = 0.046). These results suggest that renal artery blood flow velocity increases during the first postnatal week in preterm infants and is likely related to increases in aortic blood flow velocity and reduction in renal vascular resistance. (J PEDIATR 1996;129:251-7)